Surface temperatures in electron beam welding cavities in stainless steel 304 and aluminum 1100, 2024, and 6061 were measured with a narrow band infrared radiation pyrometer. A special device was designed for mounting the radiation-sensing probe next to the electron beam gun in the welding chamber. This mounting device included a mechanism for oscillating the probe so as to scan the cavity region both perpendicular and parallel to the welding direction. At the center of its movement the probe viewed almost directly down into the welding cavity. The effect of interreflections occurring in the welding cavity were accounted for by the use of an apparent spectral cavity emittance. Typical measured cavity temperature distributions for SS-304 ranged from 1950/sup 0/C at the mouth to a peak of 2350/sup 0/C at the cavity base and from 1300 to 1650/sup 0/C for A1-1100. First approximation predictions of the cavity surface temperatures were determined by assuming a quasi-steady-state condition. The surface temperature is then a function of the vapor pressure, which is required to balance the surface tension and the hydrostatic pressure both of which tend to collapse the cavity. Base temperatures thus predicted were about 5% and 10% higher than measured for SS-304more » and A1-1100, respectively. It was determined that EB welding cavity base surface temperatures are relatively constant with varying penetration depth because they are more strongly dependent on the curvature at the base than on the penetration depth. Average peak temperatures for SS-304, A1-1100, A1-6061, and A1-2024 were measured to be approximately 2300, 1700, 1525, and 1475/sup 0/C, respectively. The peak temperatures were lower for A1-6061 and A1-2024 than for A1-1100 because they contained a significant amount of magnesium and zinc, both of which have comparatively high vapor pressures. « less